The present invention relates generally to electrical stimulators for biological tissue and more particularly to output limited electrical stimulators for biological tissue.
Electrical stimulators providing an electrical stimulus signal are useful for biological tissue. One significant use for electrical stimulators of this type is for transcutaneous electrical nerve stimulation (TENS) which generate carefully controlled electrical stimulus signals which are delivered via suitable electrodes through a patient's skin to underlying biological tissue. The electrical stimulus signals are utilized for masking pain signals, for example, the sensation of pain felt by a patient after surgery. Because the patient's response to transcutaneous electrical nerve stimulation may vary significantly, a wide range of electrical stimulus signals and sets of stimulus parameters for those electrical stimulus signals must be provided. A second use of electrical stimulators is for neuromuscular (NMS) stimulation in order to initiate or control muscular contraction in a patient. Since a wide variety of muscular contractions are available, a wide variety of sets of stimulus parameters must be provided to designate a wide variety of electrical stimulus signals. Such stimulus parameters typically specify the pulse type, pulse amplitude and pulse duration in a pulsatile electrical stimulus system. A set of stimulus parameters or a plurality of sets, uniquely specify an electrical stimulus signal.
While it is desirable to have the electrical stimulus signal applied to biological tissue for transcutaneous electrical nerve stimulation or neuromuscular stimulation, there is some risk of cardiac excitation in the patient or user of the electrical stimulator. For this reason, there are generally acceptable standards relative to the "strength-duration curve" of electrical stimulus signals. Such strength-duration curves specifies the maximum strength of electrical charge which the electrical stimulus signal should be able to generate and pass through the biological tissue load. The electrical charge limit is therefore a known, predetermined standard. The electrical charge limit of the electrical stimulus signal is a function of pulse amplitude, pulse duration and pulse type (wave form).
In the operation of an electrical stimulator, it is often desirable to have the stimulation parameters varied or modulated according to some predetermined algorithm. Such variation is useful to have available the maximum possible range of stimulus parameters to gain the maximum beneficial response to transcutaneous electrical nerve stimulation (TENS) or for adjusting the strength of muscular contraction under varying loads or conditions. One technique for varying the stimulus parameters is to simultaneously specify a maximum value for stimulation parameter such as pulse amplitude and pulse duration and randomly select certain percentages of those maximum values for use in developing the electrical stimulus signal.
When such a stimulus parameter variation requirement is combined with a maximum charge limit requirement, a problem is uncovered. If the maximum charge limit is to be observed simply by selection of maximum values for pulse amplitude and pulse duration, the maximum values to be selected for these stimulus parameters must be below those which would create the maximum charge limit, for any possible combination of stimulus parameter values, even though in practice only a small percentage of the electrical stimulus signals would be above the maximum charge limit due to the random variation of the two stimulus parameters.